1. Field of the Invention
This invention relates to pneumatic controls. In particular, the invention relates to an improved electro-pneumatic control system and electronic control circuit having proportional, integral and derivative ("PID") feedback characteristics adapted to provide fast and accurate pressure regulation of a pneumatic load.
2. Description of the Related Art
In many industrial applications it is often desirable to be able to control a relatively large volume of air at a predetermined pressure and flow rate using a relatively small pneumatic or electrical control signal. Typical industrial controls for this purpose are open-loop mechanical controls, being comprised of various combinations of springs, valves, diaphragms and flow orifices. Typically, a pneumatic control signal input is provided at one end of a pneumatic regulator and operates against a spring-loaded control valve or similar device to achieve a corresponding desired pressure and/or flow output response at the output end according to well-known principles of fluid dynamics. This output pressure may be used to operate a pneumatic load, such as a hydraulic valve, piston actuator, or pneumatic motor.
Electrical or electro-pneumatic controls are also known for pneumatic pressure regulation. Typical electro-pneumatic controls are also open-loop controls adapted to provide an output pressure or flow response in accordance with an electrical control signal input. Typically, a small electrical current signal is provided to the main winding of a spring loaded solenoid, which, in turn, operates a pneumatic valve. Depending upon the magnitude of the current signal provided to the main winding of the solenoid, the displacement of the valve may be varied to determine the approximate flow rate and pressure output response provided to a pneumatic load.
A significant drawback of open-loop control systems is that the output response often varies considerably with the supply pressure and the back-pressure exerted by the load. Among other things, this can cause undesirable fluctuations in the speed and power output of air driven machinery or other pneumatically operable equipment. For static loads, such as a pneumatically operable valve, such pressure variations can cause the valve setting to drift or fluctuate from one valve setting to another, allowing too much or too little fluid to flow through the valve. This effect can be problematic in some service applications.
Providing a pneumatic control regulator having closed-loop pneumatic or electrical feedback can alleviate some of these problems. Feedback control tends to compensate for variations in load and supply pressures such that a range of selectable output pressures and/or flow rates may be provided. Most such feedback control regulators, however, have poor dynamic response, and are unable to adjust quickly to supply and load back-pressure variations. This slow response time makes such prior art feedback control regulators unsuitable for many critical service applications, such as for controlling a cooling valve in a nuclear power plant. Such applications require not only a high degree of steady-state accuracy, but also a fast response time in order to quickly respond to rapidly changing conditions.
Another significant drawback of prior art feedback control regulators is that they tend to be load dependent. Depending upon the particular size and response characteristics of the load being controlled, most prior art feedback control regulators must be adjusted or "tuned" in order to achieve acceptable dynamic response and steady-state performance. Tuning is typically accomplished by adjusting the gain of the various feedback paths of the control system. The need to adjust or tune a control circuit to a particular load, however, is undesirable in applications having changing load requirements or in critical service applications requiring a high degree of precision and stability for any given load condition.
Previous attempts to overcome the aforementioned problems have involved sophisticated control circuitry having "anti-windup" features and other high-level circuitry in order to attempt to accommodate changes in the size and response characteristics of the particular load being controlled. While some of these control circuits have been able to achieve acceptable results from a performance standpoint, the sheer complexity of the circuitry reduces the overall reliability of the system as well as increases the cost of such control systems.